The non-contact type potentiometer which is composed of a magnetic reluctance element and a permanent magnet operates with high reliability and, therefore, is used for the measurement of the angle of rotation of a part designed to produce a continuous rotary motion. The distance traveled by a wheel with a known circumference can be found, therefore, by measuring the angle of rotation of the wheel. The linear distance traveled such as by a sliding part in a machine can be found, therefore, by converting the sliding motion into a rotary motion by means of a rack and pinion and then accurately measuring the angle of rotation.
The major disadvantage suffered by this non-contact type potentiometer resides in the fact that since the measurement can be obtained only in the range of 90 degrees + 90 degrees, namely 180 degrees, the potentiometer cannot measure the rotation angle of a wheel throughout the entire circle of 360 degrees. Thus, this non-contact type potentiometer cannot be used for the measurement of the amount of continuous motion made by the wheel mentioned above. For measurement of this sort, there is generally used an encoder. Unfortunately, the encoder is heavy and expensive.